This invention relates generally to controlling defrost of evaporator coils and, more particularly, to an adaptive method of defrosting evaporator coils of a transport refrigeration system.
Transport vehicles that transport temperature sensitive cargo include a conditioned space whose temperature is controlled within a predetermined temperature range. The temperature control unit can be programmed to cool or heat the conditioned space to the thermal set point.
When in the cooling mode the temperature control unit is prone to a build-up of frost on the evaporator coil. Such frost, or eventually ice, can substantially decrease the efficiency of the unit, and therefore defrost cycles are typically applied to remove the condensate/ice. A defrost cycle can be accomplished by reversing the flow of refrigeration through the system so as to circulate a heated fluid through the evaporator coil. It may also be accomplished with the use of an electrical resistance heater. After each periodic defrost cycle, the temperature control unit is returned to operate in the cooling mode until the build-up of condensation again requires a defrost cycle.
Generally, one would like to maximize the cooling cycle times and minimize the defrost cycle times. That is, since the time during defrost represents time in which the conditioned space is not being cooled, and since following defrost, it is necessary to not only make up for the heating of the conditioned space but also to cool the evaporator coil itself after being heated up by the defrost cycle, it is preferable to wait as long as possible to initiate the defrost cycle. However, the loss of efficiency as caused by a build-up of frost on the coil, will eventually necessitate the defrost cycle being initiated. Thus for any particular unit, the times in which the defrost cycle is initiated can be optimized by determining how much condensate will be built up before initiation of the defrost cycle. Generally, because of rather stable operating conditions and parameters (i.e. fixed conditioned space temperature, fixed compressor operator speed, and fixed voltage to the resistance heater), this optimum build-up of frost is directly related to operating time and, once stabilized, one can simply, and quite consistently, initiate the defrost cycle after a predetermined time in which the compressor has run since the last defrost cycle.
In some applications however, the operating parameters of the accumulation interval are not necessarily constant. For example, in the case of refrigerated containers that are loaded on a transport ship: the payload of the container may need to be cooled-down immediately after being loaded; the humidity level inside the container may change according to characteristics of the load or according to varying temperature and humidity of air introduced into the container for the purposes of venting the cargo; and the intensity of the cooling and therefore the temperature of the evaporator coil may change according to changes in cooling demand due to diurnal cycles, weather, or changes in climate along the course of the voyage.
It has long been appreciated that adapting to changes in operating parameters may be accomplished by observing the time required to defrost the unit, comparing this time to a previously determined ideal time, and adjusting the accumulation interval to be longer or shorter according to whether the defrost time is less or greater than the ideal time.
In some applications however, the operating parameters are not necessarily constant. For example, in the case of refrigerated containers that are loaded on a transport ship, the containers are powered from the ship's system, which is not consistent in providing power at a fixed level because of the number of different power units that are periodically brought online or offline. Since the wattage varies with the square of the voltage of the ships power, the amount of heat delivered by the electrical resistance heater can vary substantially over a given period of time. This, in turn, can shorten or extend the time needed for defrost.